1. Field of the Invention
The present invention relates to a floppy disk drive which operates to travel a head carriage for magnetical recording of data in the radial direction of a magnetic disk by driving a stepping motor.
2. Description of the Related Art
This type of floppy disk drive (FDD) has been conventionally arranged to have a stepping motor, a head carriage for magnetically recording data, and a magnetic disk on which data is recorded. In operation, the stepping motor is actuated by exciting current to rotate with the controlled rotating angles so that the head carriage is allowed to travel by a predetermined distance in the radial direction of the magnetic disk.
The exciting current flows through the stepping motor for a fixed interval of time, e.g. 10 to 18 ms, after the motor receives a step signal corresponding to a FDD interface signal.
When the step signal is input, an exciting current enable signal having a constant time duration is generated, then the exciting current is allowed to flow through motor coils of the stepping motor for the above constant time duration so that the stepping motor may travel forward. When the stepping motor finishes a one-step travel, the exciting current is cut off.
If the exciting current is not large, the stepping motor moves too slowly. Hence, the current to be applied to the motor is selected larger than a certain magnitude. However, the continuous application of such large current causes a large overshoot of the stepping motor travel, which requires a considerably long time until the stepping motor is settled in a right place. To overcome this shortcoming, in general, the exciting current is temporarily cut off after a proper time is passed for the purpose of checking the action of the stepping motor and suppressing the overshooting travel of the motor from the desired location.
This type of stepping motor derives damped oscillation at application of the exciting current. The motor vibrates for a certain length of time until it is relocated to a right place. This vibration may result in bringing about noisy sounds in the stepping motor and the head carriage.
Moreover, when the exciting current is cut off, counter electromotive force is induced by the motor coil itself, so that the current is kept flowing in the coil and flows backward the power supply unit. This inverse current brings about noises in a power line which may intrude to a control circuit of the stepping motor control circuit in which those noises may rotate the stepping motor irregularly.
To suppress such ripples caused in the stepping motor, Imagawa teaches in JPA-4-133695 a control method of a stepping motor for suppressing the vibrations caused in a low-speed drive and enabling the smooth motion of the stepping motor by suppressing a driving voltage of the stepping motor according to reduction of frequency of input pulses.
Further, with regard to the current of the stepping motor, Tsuyuguchi teaches in JPA-5-20814 a control method of a stepping motor for stabilizing the current of the stepping motor by performing a PWM control during a seek period to change a duty ratio or a frequency so as to compensate a current variation in the windings resulting from variation of the supply voltage.
However, these methods require additional circuits.
By substantially reducing the exciting current applied while the damped oscillation takes place, the damped oscillation is suppressed and the time is reduced until the stepping motor is settled in the right spot. For example, a method of chopping a supply voltage for a predetermined duration after the stepping motor returns to a target location and applying the chopped voltage to the motor coil is commonly known. This method damps the damped oscillation and thereby reduces the noises.
This method, however, brings about noises by the self induction of the motor coils when the exciting current is cut off, which cause noises in the power line. These noises may be applied to the control circuit of the stepping motor, and induce irregular rotation of the stepping motor.
Sakurai has given in JPU-63-17598 a teaching of how to suppress the noises generated in the power line. Sakurai's method includes switching transistors at finish of seek so that the inverse current caused by self induction of the motor coils flows through additional resistors connected to the power supply unit of the stepping motor. This reduces reflux of the current to the power supply, thereby suppressing the noises generated in the power line.
The method given by Sakurai, however, needs additional circuits having resistors and transistors.